Proportional control valves include actuators that apply an adjustable force to a directional control valve connecting a closed volume load with both supply and exhaust fluid flow paths. The directional control valve reacts to the adjustable force by directing fluid through one of the two fluid flow paths until an equilibrium is reached with a reactionary force applied by the working pressure. For example, when the adjustable force is greater than the reactionary force, the supply path is opened and the exhaust path is closed. Conversely, when the adjustable force is less than the reactionary force, the supply path is closed and the exhaust path is opened. However, both paths are closed when the two forces are equal.
One way of generating the adjustable force is by applying a pilot pressure to one face of a piston that operates the valve. The reactionary force is generated by applying the working pressure to an opposing face of the piston. The piston is movable in opposite directions by differences between the pilot and working pressures and is balanced against movement when the two pressures are equal.
However, the pilot pressure itself is difficult to accurately regulate and limits the response time of proportional control valves for varying the working pressure, especially when the source of pilot pressure is located at a distance from the proportional control valve. Some uses for proportional control valves require much faster response times. For example, anti-lock braking systems require faster response times including pulsed responses to maintain maximum braking force over varying traction conditions without significant wheel skid.
U.S. Pat. No. 2,194,762 to Maliphant discloses an early attempt to replace pilot pressure with a solenoid actuator to regulate brake pressure. The solenoid actuator applies an adjustable force against a piston that isolates a working pressure of a brake cylinder from an exhaust pressure. In other words, the adjustable force applied by the solenoid actuator is opposed by a reactionary force applied by the working pressure against the piston. The piston has a very large area exposed to the working pressure, and the solenoid must be very large to match reactionary forces corresponding to a range of working pressures. In addition, a valve seating force generated by a supply pressure also opposing the adjustable force distorts the valve's performance.
U.S. Pat. No. 4,557,527 to Stumpe and U.S. Pat. No. 5,015,046 to Bissell disclose more recent examples of proportional control valves which supplement pilot pressures with electromagnetic actuators for regulating braking pressure. However, Stumpe's electromagnetic actuator has just two excited positions that are selected by an electronic circuit on the basis of a signal provided by a remote pressure sensor. Bissell's actuator is a stepper motor combined with a threaded thrust member and a spring for converting rotary positions of the motor into predetermined forces against a piston. Pilot pressure is also applied to the piston. Therefore, both a measurement of the pilot pressure and a summing calculation are required to determine the total adjustable force against the piston. In addition, the piston has a very large area exposed to the working pressure, and a correspondingly large amount of electrical power is required to move the piston with the stepper motor against the reactionary forces.
My U.S. Pat. No. 5,123,718 discloses a proportional control valve that regulates brake pressure proportional to an adjustable force applied by a solenoid actuator. A directional control valve connecting a brake to both supply and exhaust fluid flow paths is made up of an inlet valve that regulates a flow of fluid from a supply port to a working (delivery) port, and an outlet (control) valve that regulates a flow of fluid from the working port to a primary exhaust port.
The solenoid actuator of my prior valve is sized to apply an adjustable force proportional to a predetermined range of working pressures applied over an area enclosed by a seat of the outlet valve. A product of the working pressure and the area of the outlet valve seat is equal to a reactionary force that opposes the adjustable force. An outlet valve poppet attached to the solenoid actuator is urged by the adjustable force in a first direction for opening the inlet valve and closing the outlet valve and is urged by the reactionary force in a second direction for closing the inlet valve and opening the outlet valve.
The area enclosed by the outlet valve seat is limited to reduce the size of the solenoid that is required to match the reactionary forces. However, the limited area of the outlet valve seat also restricts the rate of fluid flow along the exhaust path to the primary exhaust port. Accordingly, a separate exhaust path is provided through a dump valve to a secondary exhaust port for discharging higher rates of fluid flow. In addition, a vent valve is used to distinguish between high and low rates of fluid flow for controlling operation of the dump valve.